The recent proliferation of microwave ovens for the preparation and cooking of food has created a need for the production of improved packaging in order to render certain types of food more amenable to microwave cooking. Thus, for example, certain foods, such as popcorn, may not absorb enough microwave energy to generate sufficient heat to pop or cook. Other foods require browning or crisping of their surfaces, results which cannot ordinarily be achieved by the use of conventional food packaging compositions in microwave ovens.
In order to meet this need for improved microwaveable packing, several different approaches have been proposed.
One general approach has been to form a multi-layered wrap-type composition composed of an energy absorbing susceptor material and a plastic film or other dielectric substrate. Thus, for example, U.S. Pat. No. 4,267,420 (Brastad) discloses a packaging material which is a plastic film or other dielectric substrate having a thin semiconducting coating, preferably of evaporated aluminum. Somewhat similarly, U.S. Pat. No. 4,434,197 (Petriello et al) shows a multi-layered laminated microwaveable packaging material including outside layers of polytetrafluoroethylene, two intermediate layers of pigmented polytetrafluoroethylene and a central layer of polytetrafluoroethylene having dispersed therein particles of an energy absorbing susceptor material such as graphite, ferric oxide or carbon.
A second general approach which has been proposed involves the dispersion of particles of a microwave absorbing composition in a polymeric or ceramic-type material matrix. Thus, for example, U.S. Pat. No. 4,190,757 (Turpin et al) discloses a microwaveable package composed of a non-lossy dielectric sheet material defining a container body and a lossy microwave absorbitive heating body connected thereto, such heating body typically comprising particles of microwave absorbitive susceptor material (including zinc oxide, germanium oxide, iron oxide, alloys of metals such as of manganese, aluminum and copper, oxides, carbon and graphite) in a ceramic-type binder (such as cement, plaster of paris or sodium silicate). Somewhat similarly, U.S. Pat. No. 4,518,651 (Wolfe) shows microwaveable composite materials comprising a polymeric matrix having electronically conductive particles dispersed therein, which matrix is bound to a porous substrate. This patent teaches that it is critical that at least some of the polymer matrix beneath the surface of the substrate be substantially free of electronically conductive particles and be intermingled with the substrate.
European Patent Publication 242,952 discloses a microwaveable packaging material which is a composite comprising a dielectric material (e.g., polyethylene terephthalate film) coated with a mixture of an electrically conductive metal or metal alloy in flake form in a dielectric matrix. This patent indicates that to obtain optimum heating performance reproductibility, circular flakes with flat surfaces and smooth edges should be employed. Somewhat similarly, U.S. Pat. No. 4,866,232 (Stone) discloses a food package for use in a microwave oven, such package being produced by the deposition of a metallized ink consisting of metal particles suspended in an ink-like substance onto a container formed from a heat resistant material which is pervious to microwaves.
While many of the above and similar microwaveable packaging compositions will function to convert microwave energy into heat, there is still a need for improved packaging materials. Thus, many proposed microwaveable packaging materials tend to heat uncontrollably in a microwave oven, leading to charring or even arcing, ignition and burning of the packaging material. Other materials are not capable of generating sufficient heat quickly, while several materials, while functioning desirably, are economically prohibitive for widespread use.
Accordingly, it is an object of this invention to provide a microwaveable package which provides for increased control of the heat generated by exposure to microwaves.
It is a further object of this invention to provide a microwaveable packaging ink composition which when deposited on a dielectric substrate will offer improved control of the heat generated upon exposure to microwave radiation.
It is yet a further object of this invention to provide a microwaveable packaging ink composition which can be economically employed.
It is an additional object of this invention to provide a method of economically producing a microwaveable package which provides increased control of the heat generated upon exposure to microwave radiation.
These objects, and other additional objects, will become more fully apparent from the following description and accompanying Examples.